Catalyst for purifying exhaust gases

ABSTRACT

A catalyst for purifying exhaust gases which aims to efficiently purify NO x  in exhaust gases in oxygen-rich atmospheres whose oxygen concentrations are more than required for oxidizing CO and HC, and to attain improved NO x  purifying performance after used for a long time. The catalyst includes, being arranged in an order from the upstream to the downstream of exhaust gas flow, a first catalyst  1  in which a noble metal catalyst  12  is loaded on a porous acidic support, a second catalyst  2  in which at least one kind of metal  22  selected from the group consisting of alkali metals, alkaline-earth metals, and rare earth metals is loaded on a porous support, and a third catalyst  3  in which a noble metal catalyst  32  is loaded on a porous support. Since SO 2  is not adsorbed or oxidized by the first catalyst  1 , SO 2  is discharged downstream from the second catalyst  2  without generating sulfate. Therefore, since the NO x  absorber is free from degradation even after used for a long time, the second catalyst  2  maintains high NO x  adsorbing performance on the fuel-lean side, and the third catalyst  3  reduces NO x  which are emitted from the second catalyst  2  on the stoichiometric point or on the fuel-rich side.

This is a Continuation of application Ser. No. 08/625,922 filed Apr. 1, 1996 now U.S. Pat. No. 5,849,254, which in turn is a Continuation Application of application Ser. No. 08/366,506 filed Dec. 30, 1994, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a catalyst for purifying exhaust gases from automobiles, and more particularly, to a catalyst which can efficiently purify nitrogen oxides (NO_(x)) in exhaust gases in oxygen-rich atmospheres whose oxygen concentrations are more than required for oxidizing carbon monoxide (CO) and hydrocarbons (HC) therein.

2. Description of the Related Art

As catalysts for purifying automotive exhaust gases, there have been employed 3-way catalysts so far which oxidize CO and HC and reduce NO_(x) simultaneously into innocuous entities. For example, 3-way catalysts have been known widely which comprise a heat resistant support formed of cordierite, a catalyst carrier layer formed of gamma-alumina and disposed on the support, and a noble metal catalyst selected from the group consisting of Pt, Pd, and Rh and loaded on the catalyst carrier layer.

The purifying performance of these 3-way catalysts for purifying exhaust gases largely depends on the air-fuel ratio (A/F) of automotive engines. For instance, when the air-fuel ratio is large, i.e., when the fuel concentration is small (or on the fuel-lean side), the oxygen concentration is large in exhaust gases. Accordingly, the reactions of oxidizing CO and HC are active, but the reactions of reducing NO_(x) are inactive. On the other hand, when the air-fuel ratio is small, i.e., when the fuel concentration is high (or on the fuel-rich side), the oxygen concentration is small in exhaust gases. Accordingly, the oxidation reactions are inactive, but the reduction reactions are active.

Moreover, when driving automobiles, especially when driving automobiles in urban areas, the automobiles are accelerated and decelerated frequently. Accordingly, the air-fuel ratio varies frequently in a range from the values adjacent to the stoichiometric point (or the ideal air-fuel ratio) to the fuel-rich side. In order to satisfy the low fuel consumption requirement during the driving in urban areas, it is necessary to operate automobiles on the fuel-lean side where the air-fuel mixture containing as excessive oxygen as possible is supplied to the engines. Hence, it has been desired to develop a catalyst which is capable of adequately purifying NO_(x) on the fuel-lean side (i.e., in the oxygen-rich atmospheres).

In view of the aforementioned circumstances, the applicants of the present invention applied for a Japanese Patent for a novel catalyst under Japanese Unexamined Patent Publication (KOKAI) No.5-317,652. This catalyst comprises a support, and an alkaline-earth metal and Pt loaded on the support. With this support, NO_(x) are adsorbed on the alkaline-earth metal when the air-fuel ratios are on the fuel-lean side (i.e., in the oxygen-rich atmospheres) driving. Then, when the automobiles are driven under the air-fuel ratios at the stoichiometric point or on the fuel-rich side (i.e., in the oxygen-lean atmospheres), NO_(x) are purified by reacting with reducing gases such as HC and the like. Thus, this catalyst is superb in the NO_(x) purifying performance even on the fuel-lean side (i.e., in the oxygen-rich atmospheres).

This catalyst is believed to adsorb NO_(x) by the following mechanism: The alkaline-earth metal, for example, Ba is loaded on the support in a form of the simple oxide. The simple oxide of Ba reacts with NO_(x) to produce barium nitrate (Ba(NO₃)₂). Thus, NO_(x) are adsorbed on the support of the catalyst.

However, the exhaust gases usually contain SO₂ which is produced by burning sulfur (S) contained in the fuel. The catalyst ingredient further oxidizes SO₂ to SO₃ in the oxygen-rich atmospheres (i.e., on the fuel-lean side). Then it has become apparent that SO₃ reacts readily with water also contained in the exhaust gases to produce sulfuric acid, and that the resulting sulfuric acid reacts with Ba to produce sulfite and/or sulfate, which do not adsorb NO_(x). When Ba or the like is converted into sulfite or sulfate, it cannot adsorb NO_(x) any more. As a result, this catalyst suffers from a drawback in that it is deteriorated in terms of the NO_(x) purifying performance after it has been used for a long time.

SUMMARY OF THE INVENTION

The present invention has been developed in view of the aforementioned circumstances. It is therefore an object of the present invention to provide a catalyst which can efficiently purify nitrogen oxides (NO_(x)) in exhaust gases in oxygen-rich atmospheres whose oxygen concentrations are more than required for oxidizing CO and HC. It is an another object of the present invention to provide a catalyst which attains improved NO_(x) purifying performance after it has been used for a long time by preventing an NO_(x) absorber from degraded due to its generation of salts of SO_(x).

A catalyst for purifying exhaust gases according to a first aspect of the present invention comprises, being arranged in an order from the upstream to the downstream of exhaust gas flow, a first catalyst in which a noble metal catalyst is loaded on a porous acidic support, a second catalyst in which at least one kind of NO_(x) absorber selected from the group consisting of alkali metals, alkaline-earth metals, and rare earth metals is loaded on a porous support, and a third catalyst in which a noble metal catalyst is loaded on a porous support.

A catalyst for purifying exhaust gases according to a second aspect of the present invention is a catalyst in which at least one kind of NO_(x) absorber selected from the group consisting of alkali metals, alkaline-earth metals and rare earth metals, and a noble metal catalyst are loaded on a porous support, and the surface layer of the porous support includes oxides of at least one metal selected from the group consisting of iron (Fe), cobalt (Co), nickel (Ni), copper (Cu) and manganese (Mn).

In the catalyst for purifying exhaust gases according to the first aspect of the present invention, first, automotive exhaust gases contact the first catalyst. This first catalyst has a property of not adsorbing SO₂, because the support is acidic. Therefore, SO₂ is prevented from being adsorbed on the support and from being oxidized by the noble metal catalyst, and most SO₂ flows into the second catalyst as it is. When a metal such as nickel (Ni), iron (Fe), zinc (Zn), and vanadium (V) is loaded in addition to the noble metal catalyst, SO₂ is more securely prevented from oxidation. This is supposed to be because these metals serve to decrease the oxidation force of the noble metal catalyst.

The porous acidic support of the first catalyst is suitably formed from materials such as SiO₂, ZrO₂, SiO₂—Al₂O₃, TiO₂, and so on.

The exhaust gases flowing into the second catalyst contain almost no SO₃ for the reason described in the above. Therefore, at least one kind of NO_(x) absorber selected from the group consisting of alkali metals, alkaline-earth metals, and rare earth metals is prevented from generating salts with SO₃, and can adsorb NO_(x) in the exhaust gases efficiently. It has been apparent that NO₂ is better adsorbed by NO_(x) absorbers than NO. NO in the exhaust gases in oxygen-rich atmospheres is efficiently oxidized by the first catalyst and flows into the second catalyst in the form of NO₂ and other NO_(x). Therefore, the NO_(x) absorber in the second catalyst efficiently adsorbs NO_(x). In this way, the exhaust amounts of NO_(x) in the fuel-lean atmosphere are decreased.

Then, the NO_(x) adsorbed by the second catalyst is discharged when the air-fuel ratio is at the stoichiometric point or on the fuel-rich side, and flows into the third catalyst where the NO_(x) is reduced into innocuous entity by 3-way activity.

Examples of the suitable porous supports of the second catalyst and the third catalyst include Al₂O₃, zeolite, SiO₂, ZrO₂, and SiO₂—Al₂O₃. Al₂O₃ is most preferable. The supports can be formed from these porous bodies in themselves, or formed by coating honeycombs made of cordierite, heat-resistant metals or the like with these porous bodies.

In the second catalyst, it is also preferable to load a transition metal in addition to at least one kind of NO_(x) absorber selected from the group consisting of alkali metals, alkaline-earth metals, and rare earth metals. This transition metal may be loaded by adding not only as a simple substance but also as a composite oxide such as BaCuO₂, LaCoO₃, LaBaCoO₃. The loading of this transition metal allows the temperature window of NO_(x) purifying performance to be transferred in the direction of higher temperatures.

For example, when Ba alone is loaded, the peak temperature of NO_(x) purifying performance is 250° C., but when Ba and Cu are loaded, the peak temperature moves to 400° C., and when Ba and Co are loaded, the peak temperature moves to 450° C., and when Ba and Fe are loaded, the peak temperature moves to 400° C. Further, the loading of plural kinds of metals achieves an increase in the width of the temperature window. This offers an advantage of providing greater freedom in installing catalysts. It must be noted that transition metals exhibit lower oxidization catalyst performance than noble metal catalysts. Therefore, almost no SO₃ generates in the second catalyst, and at least one kind of NO_(x) absorber selected from the group consisting of alkali metals, alkaline-earth metals, and rare earth metals is able to exhibit the function of adsorbing NO_(x) to the maximum.

In the catalyst for purifying exhaust gases according to the second aspect of the present invention, the surface layer of the porous support includes oxides of at least one metal selected from the group consisting of Fe, Co, Ni, Cu and Mn. These metal oxides have the function of oxidizing SO₂ into SO₃ although this function is not strong, and the metal oxides take in SO₃ and H₂SO₄ which is formed by reacting SO₃ with water, and form salts of SO_(x). Therefore, SO_(x) do not reach the NO_(x) absorber which exists inside of the support and accordingly the NO_(x) absorber is prevented from generating salts of SO_(x).

The above metal oxides generate salts of SO_(x) and NO_(x), respectively, but the salts of NO_(x) decompose at about 300° C. and the salts of SO_(x) decompose at about 500° C. Therefore, at the time of ordinary driving where the inlet automotive exhaust gas temperature to the bed of catalyst is approximately in a range from 300 to 400° C., the NO_(x) which were taken in and formed into salts decompose soon and are taken in by the NO_(x) absorber on the inside of the porous support. Further, the metal oxides which once decomposed and regenerated adsorb SO_(x) and form salts of SO_(x). On the other hand, at the time of ordinary driving, SO_(x) exist in the form of salts, and accordingly, do not react with the NO_(x) absorber. Therefore, the NO_(x) absorber is prevented from being degraded due to its generation of salts of SO_(x), and the salts of metal oxide and SO_(x) decompose approximately above 500° C. and SO_(x) are discharged downstream.

It is necessary that SO₂ and NO are oxidized into SO₃ and NO₂ respectively in order to react with the metal oxides and generate salts. In the second aspect of the invention, a noble metal catalyst, when loaded on the surface layer in addition to the metal oxides, promotes the oxidation of NO and the resulting NO₂ reacts with the metal oxides and generates salts. Accordingly, NO is prevented from being discharged downstream. In the meanwhile, the noble metal catalyst also promotes the oxidation of SO₂, and the taking in of the resulting SO_(x) by the above metal oxides is given preference over other elements. As described in the above, NO₂ is quickly taken in by the NO_(x) absorber and the taken-in NO_(x) securely react with HC or CO when the fuel-air ratio is at the stoichiometric point. As a result, the NO_(x) purifying performance even after used for a long time is improved.

The catalyst for purifying exhaust gases according to the present invention has the following advantages.

According to the present invention, at least one kind of loaded NO_(x) absorber selected from the group consisting of alkali metals, alkaline-earth metals, and rare earth metals is prevented from contacting SO₃. Therefore, the NO_(x) absorber is free from degradation caused by its generation of sulfite and/or sulfate. Therefore, the catalyst exhibits high NO_(x) purifying performance even after it has been used for a long time and is superior in durability.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the present invention and many of its advantages will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings and detailed specification, all of which forms a part of the disclosure:

FIG. 1 is a perspective view of a catalyst for purifying exhaust gases according to a first preferred embodiment of the present invention;

FIG. 2 is a cross-sectional view for explaining the constitution of the catalyst for purifying exhaust gases according to the first preferred embodiment of the present invention; and

FIG. 3 is a cross-sectional view for explaining the constitution of a catalyst for purifying exhaust gases according to a fifth preferred embodiment of the present invention.

THE PREFERRED EMBODIMENTS OF THE PRESENT INVENTION

Having generally described the present invention, a further understanding can be obtained by reference to the specific preferred embodiments which are provided herein for purposes of illustration only and are not intended to limit the scope of the appended claims.

The First Preferred Embodiment

FIG. 1 and FIG. 2 show a catalyst for purifying exhaust gases according to a first preferred embodiment of the present invention. This catalyst comprises a first catalyst 1, a second catalyst 2, and a third catalyst 3, being arranged axially. The respective catalysts have the same diameters and the axial length of the first catalyst, the second catalyst and the third catalyst has a ratio of 2:6:2. This catalyst is used by arranging the first catalyst 1 in the upper stream of the exhaust gases.

The first catalyst 1 comprises a first honeycomb support 10 formed of cordierite, a first catalyst carrier layer 11 formed of SiO₂ and coated on the surface of the first honeycomb support 10, and platinum (Pt) 12 loaded on the first carrier layer 11.

The second catalyst 2 comprises a second honeycomb support 20 formed of cordierite, a second catalyst carrier layer 21 formed of Al₂O₃ and coated on the surface of the second honeycomb support 20, and barium (Ba) 22 loaded on the second carrier layer 21.

The third catalyst 3 comprises a third honeycomb support 30 formed of cordierite, a third catalyst carrier layer 31 formed of Al₂O₃ and coated on the surface of the third honeycomb support 30, and platinum (Pt) 32 and Rh (rhodium) 33 loaded on the third catalyst layer 31.

Now, we will describe a method of producing this catalyst instead of describing its constitution in detail. It must be noted that hereinafter, parts mean parts by weight.

<The First Catalyst>

100 parts of porous silica powder, 70 parts of silica sol containing silica in an amount of 10% by weight, and 30 parts of water were stirred, thereby preparing slurry for coating.

Next, the first honeycomb support 10 formed of cordierite was immersed in water and blown to blow away excessive water. Then the first support 10 was immersed in this slurry and blown to blow away an excess of the slurry, dried at 80° C. for 20 minutes, and calcined at 600° C. for one hour, thereby forming the first carrier layer 11 on the support 10. The SiO₂ coating was in an amount of 120 grams with respect to 1 liter of the honeycomb support 10.

The honeycomb support thus coated with SiO₂ was immersed into an aqueous bivalent platinum ammine hydroxide solution. After pulled up and blown to blown away water droplets in excess, it was calcined at 250° C. for one hour. The loading amount of platinum (Pt) was 2.0 grams with respect to 1 liter of the honeycomb support 10.

<The Second Catalyst>

100 parts of alumina powder, 70 parts of alumina sol containing alumina in an amount of 10% by weight, 15 parts of a 40% by weight solution of aluminum nitrate in water, and 30 parts of water were mixed, thereby preparing slurry for coating.

This slurry was coated on the second honeycomb support 20 in the same way as in the first catalyst, thereby forming the second catalyst carrier layer 21. The Al₂O₃ coating was in an amount of 120 grams with respect to 1 liter of the honeycomb support 20.

The honeycomb support coated with Al₂O₃ was immersed in an aqueous barium acetate solution. After pulled up and blown to blow away water droplets in excess, it was dried at 110° C. for 12 hours and calcined at 600° C. for one hour. The loading amount of barium (Ba) 22 was 0.3 mol with respect to 1 liter of the honeycomb support 20.

<The Third Catalyst>

100 parts of alumina powder, 70 parts of alumina sol containing alumina in an amount of 10% by weight, 15 parts of a 40% by weight solution of aluminum nitrate in water, and 30 parts of water were mixed, thereby preparing slurry for coating.

This slurry was coated on the third honeycomb support 30 in the same way as in the first catalyst, thereby forming the third carrier layer 31. The Al₂O₃ coating was in an amount of 120 grams with respect to 1 liter of the honeycomb support 30.

The honeycomb support coated with Al₂O₃ was immersed in an aqueous platinum dinitrodiammine solution. After pulled up and blown to blow away water droplets in excess, it was calcined at 250° C. for one hour. Next, it was immersed in an aqueous rhodium nitrate solution. After pulled up and blown to blow away water droplets in excess, it was calcined at 250° C. for one hour. The loading amount of platinum (Pt) 32 was 2.0 grams with respect to 1 liter of the honeycomb support 30, and the loading amount of rhodium (Rh) 33 was 2.0 grams with respect to 1 liter of the honeycomb support 30.

The first catalyst 1, the second catalyst 2 and the third catalyst 3 thus produced were axially arranged in this order without providing space therebetween. Thus, the catalyst according to the first preferred embodiment of the present invention was produced.

The Second Preferred Embodiment

<The First Catalyst>

100 parts of porous silica powder, 70 parts of silica sol containing silica in an amount of 10% by weight, and 30 parts of water were stirred, thereby preparing slurry for coating.

Next, a honeycomb support formed of cordierite was immersed in water, and after blown to blow away excessive water, it was immersed in this slurry. After pulled up and blown to blow away the slurry in excess, it was dried at 80° C. for 20 minutes and calcined at 600° C. for one hour. The SiO₂ coating was in an amount of 120 grams with respect to 1 liter of the honeycomb support.

The honeycomb support thus coated with SiO₂ was immersed in an aqueous bivalent platinum ammine hydroxide solution. After pulled up and blown to blow away water droplets in excess, it was calcined at 250° C. for one hour. The loading amount of Pt was 2.0 grams with respect to 1 liter of the honeycomb support. The production method described so far was the same as the method of producing the first catalyst of the first preferred embodiment.

Next, the amount of water to be absorbed by the obtained honeycomb support was measured, and an aqueous nickel acetate solution was prepared to have concentration at which that amount of water absorption contained nickel by 3% by weight based on the honeycomb support. Then the honeycomb support was immersed in this solution. After pulled up and blown to blow away water droplets in excess, it was dried at 110° C. for 12 hours and calcined at 600° C. for one hour, thereby loading nickel and forming the first catalyst of the second preferred embodiment of the present invention.

<The Second Catalyst>

100 parts of alumina powder, 70 parts of alumina sol containing alumina in an amount of 10% by weight, 15 parts of a 40% by weight solution of aluminum nitrate in water, and 30 parts of water were commingled, thereby preparing slurry for coating.

This slurry was coated on the honeycomb support in the same way as in the first preferred embodiment. The Al₂O₃ coating was in an amount of 120 grams with respect to 1 liter of the honeycomb support.

This honeycomb support coated with Al₂O₃ was immersed in an aqueous copper acetate solution. After pulled up and blown to blow away water droplets in excess, it was dried at 110° C. for 12 hours and calcined at 600° C. for three hours, thereby loading copper (Cu). The loading amount of Cu was 8% by weight based on the alumina coating.

The obtained honeycomb support was further immersed in an aqueous barium acetate solution. After pulled up and blown to blow away water droplets in excess, it was dried at 110° C. for 12 hours and calcined at 600° C. for one hour. The loading amount of Ba was 0.3 mol with respect to 1 liter of the honeycomb support.

<The Third Catalyst>

This was produced in the same way as the third catalyst of the first preferred embodiment.

The Third Preferred Embodiment

<The First Catalyst>

This was produced in the same way as the first catalyst of the second preferred embodiment, except that slurry for coating was prepared by mixing 100 parts of zirconia powder, 70 parts of zirconia sol containing zirconia in an amount of 10% by weight, and 30 parts of water.

<The Second Catalyst>

This was produced in the same way as the second catalyst of the second preferred embodiment.

<The Third Catalyst>

This was produced in the same way as the third catalyst of the second preferred embodiment.

The Fourth Preferred Embodiment

<The First Catalyst>

This was produced in the same way as the first catalyst of the second preferred embodiment.

<The Second Catalyst>

This was produced in the same way as the second catalyst of the second preferred embodiment, except that slurry for coating was prepared by mixing 60 parts of alumina powder, 70 parts of alumina sol containing alumina in an amount of 10% by weight, 15 parts of a 40% by weight solution of aluminum nitrate in water, 30 parts of water, and 100 parts of YBa₂Cu₃O₇ powder. The Al₂O₃ coating was in an amount of 60 grams with respect to 1 liter of the honeycomb support.

<The Third Catalyst>

This was produced in the same way as the third catalyst of the second preferred embodiment.

The First Comparative Example

100 parts of alumina powder, 70 parts of alumina sol containing alumina in an amount of 10% by weight, 15 parts of a 40% by weight solution of aluminum nitrate in water, and 30 parts of water were mixed, thereby preparing slurry for coating.

A honeycomb support formed of cordierite was immersed in this slurry. After blown to blow away the slurry in excess and dried, it was calcined at 600° C. for one hour, thereby forming an alumina coating layer. The Al₂O₃ coating was in an amount of 120 grams with respect to 1 liter of the honeycomb support.

The honeycomb support having this alumina coating layer was immersed in an aqueous platinum dinitrodiammine solution. After blown to blow away water droplets in excess, it was dried at 250° C., thereby loading platinum (Pt). The loading amount of Pt was 2.0 grams with respect to 1 liter of the honeycomb support.

Next, this Pt-loaded honeycomb support was immersed in an aqueous barium acetate solution having a predetermined concentration. After blown to blow away water droplets in excess and dried, it was calcined at 600° C. for one hour, thereby preparing a comparative catalyst. The loading amount of Ba was 0.3 mol with respect to 1 liter of the support.

Examination and Evaluation

Each of the present catalysts and the comparative catalyst was disposed in an exhaust line of an actual vehicle equipped with a lean burn engine which had a displacement of 1.6 liters. The vehicle was driven in the urban area running modes, i.e., the 10-mode and the 15-mode, thereby examining the present catalysts and the comparative catalyst for the conversion of CO, HC, and NO_(x).

After the aforementioned evaluation, each of the present catalysts and the comparative catalyst was subjected to a bench test on durability which utilized the same type of engine as above. That is to say, each of them was disposed in an exhaust line of the engine, and then the engine was run for 50 hours while adjusting the inlet exhaust gas temperature to the bed of catalyst at 650° C. and keeping the ratio of air to fuel at 18. After this bench test was over, each of them was again disposed in the exhaust line of the actual vehicle. The vehicle was driven in the same manner as described above, thereby examining the present catalysts and the comparative catalyst for the conversion of CO, HC, and NO_(x) after the durability test. The fuel employed included sulfur in an amount of 70 ppm. The test results are shown in Table 1.

TABLE 1 Initial Conversion After Coating Loaded Conversion (%) Durability Test (%) Catalyst Layer Metal NO_(x) HC CO NO_(x) HC CO Example 1 1st SiO₂ Pt 40 93 100 36 89 92 2nd Al₂O₃ Ba 3rd Al₂O₃ Pt, Rh Example 2 1st SiO₂ Pt, Ni 65 95 100 57 89 94 2nd Al₂O₃ Ba, Cu 3rd Al₂O₃ Pt, Rh Example 3 1st ZrO₂ Pt, Ni 66 96 100 58 90 98 2nd Al₂O₃ Ba, Cu 3rd Al₂O₃ Pt, Rh Example 4 1st SiO₂ Pt, Ni 62 92  99 55 86 93 2nd Al₂O₃ YBa₂Cu₃O₇ 3rd Al₂O₃ Pt, Rh Comparative Al₂O₃ Pt, Ba 93 95 100 50 88 98 Example 1

It is appreciated from Table 1 that although the initial NO_(x) conversion performance of the present catalysts was not as good as that of the comparative catalyst, the ratios of the NO_(x) conversion performance after the durability test to the initial NO_(x) conversion performance (the NO_(xc) conversion performance after the durability test/the initial NO_(x) conversion performance) of the present catalysts were superior to that of the comparative catalyst by far. That is to say, the present catalysts had NO_(x) conversion performance less deteriorated by the durability test, and were superior in durability to the comparative catalyst.

The Fifth Preferred Embodiment

FIG. 3 shows an enlarged cross-sectional view of a part of catalyst for purifying exhaust gases according to a fifth preferred embodiment of the present invention. This catalyst comprises a honeycomb support 4, an inner catalyst carrier layer 5 formed on the surface of the honeycomb support 4, an outer catalyst carrier layer 6 formed on the surface of the inner carrier layer 5. Platinum (Pt) 50 and barium (Ba) 51 are loaded on the inner carrier layer 5, and Fe₂O₃ 60 is loaded on the outer carrier layer 6.

This catalyst for purifying exhaust gases was produced as follows. First, 150 g of water and 350 g of alumina sol containing alumina in an amount of 10% by weight were added to 500 g of alumina powder and stirred, thereby preparing slurry A. In the meanwhile, 175 g of alumina sol containing alumina in an amount of 10% by weight was added to 250 g of alumina powder and 125 g of Fe₂O₃ powder, and stirred, thereby preparing slurry B.

Next, a honeycomb support formed of cordierite was immersed in slurry A. After pulled up and blown to blow away the slurry in excess, it was dried at 80° C. for twenty minutes and calcined at 500° C. for one hour, thereby forming the inner carrier layer 5 by 100 g per liter. Then it was immersed in an aqueous platinum dinitrodiammine solution. After pulled up and blown to blow away water droplets in excess, it was dried at 80° C. and calcined at 500° C. Further it was immersed in an aqueous barium acetate solution, and pulled up and blown to blow away water droplets in excess. Then it was dried at 80° C. and calcined at 500° C. Thus, Pt and Ba were loaded on the inner carrier layer 5.

After that, the support 4 having the above inner carrier layer 5 thereon was immersed in slurry B. After pulled up and blown to blow away the slurry in excess, it was dried at 80° C. for twenty minutes and calcined at 500° C. for one hour, thereby forming the outer carrier layer 6 by 50 g per liter.

As shown in Table 2, the catalyst thus obtained had Pt in an amount of 1.0 g/liter and Ba in an amount of 0.2 mol/liter on the inner carrier layer 5, and Fe in an amount of 0.2 mol/liter on the outer carrier layer 6.

The Sixth Preferred Embodiment

This was produced in the same way as the catalyst of the fifth preferred embodiment, except that La was loaded instead of Ba, and MnO₂ was loaded instead of Fe₂O₃.

The Seventh Preferred Embodiment

This was produced in the same way as the catalyst of the fifth preferred embodiment, except that the outer carrier layer 6 was formed from slurry comprising 100 g of iron oxide powder and 100 g of alumina sol containing alumina in an amount of 10% by weight. In other words, the outer carrier layer 6 mainly comprised iron oxide and included alumina only by about 10% by weight.

The Eighth and Ninth Preferred Embodiments

These were produced in the same way as the catalyst of the seventh preferred embodiment, except that the kinds and amounts of loading metals are those shown in Table 2.

The Tenth Preferred Embodiment

250 g of alumina powder and 125 g of Fe₂O₃ powder were added to an aqueous platinum dinitrodiammine solution and stirred, thereby loading Pt on the powder. After filtered, it was dried and calcined, thereby obtaining Pt-loaded powder. 150 g of water and 350 g of alumina sol containing alumina in an amount of 10% by weight were added to 375 g of this Pt-loaded powder, and stirred, thereby obtaining slurry B. The outer carrier layer 6 was made from slurry B by 50 g/liter. A catalyst of the tenth preferred embodiment was produced in the same way as that of the fifth preferred embodiment, except the aforementioned facts. That is to say, in this catalyst, Pt in addition to Fe was loaded on the outer carrier layer 6 by 0.3 g/liter.

The Eleventh to Eighteenth Preferred Embodiments

These were produced in the same way as the catalyst of the tenth preferred embodiment, except that the kinds and amounts of loading metals were those shown in Table 2.

The Second to Fourth Comparative Examples

These were produced in the same way as the first comparative catalyst, except that the kinds and amounts of loading metals were those shown in Table 2.

Examination and Evaluation

Each of the present catalysts and the comparative catalysts was disposed in an exhaust line of an actual vehicle equipped with a lean burn engine which had a displacement of 1.6 liters. The vehicle was driven in the urban area running modes, i.e., the 10-mode and the 15-mode, thereby examining the present catalysts and the comparative catalysts for the conversion of CO, HC and NO_(x).

After the aforementioned evaluation, each of the present catalysts and the comparative catalysts was subjected to a bench test on durability which utilized the same type of engine as above. That is to say, each of them was disposed in an exhaust line of the engine, and then the engine was run for 50 hours while adjusting the inlet exhaust gas temperature to the bed of catalyst at 650° C. and keeping the ratio of air to fuel at 18. After this bench test was over, each of them was again disposed in the exhaust line of the actual vehicle. The vehicle was driven in the same manner as described above, thereby examining the present catalysts and the comparative catalysts for the conversion of CO, HC, and NO_(x) after the durability test. The fuel employed included sulfur in an amount of 70 ppm. The test results are shown in Table 2.

TABLE 2 Inner Carrier Load Amount of Outer Carrier Load Amount of Initial Conversion after Layer Element (g, mol/L) Layer Element (g, mol/L) conversion (%) Durability Test (%) Pt Pd Ba La Li K Na Pt Pb Mn Fe Co Ni Cu NO_(x) HC CO NO_(x) HC CO Examples  5 1.0 — 0.2 — — — — — — — 0.2 — — — 92 96  98 70 96 99  6 1.0 — — 0.2 — — — — — 0.2 — — — — 87 98 100 69 95 99  7 1.0 — 0.2 — — — — — — — 0.2 — — — 93 96 100 60 94 97  8 1.0 — — 0.3 — — — — — — — 0.3 — — 90 99 100 58 97 98  9 1.0 — — — 0.2 — — — — — — — 0.3 — 88 96 100 57 94 98 10 1.0 — 0.2 — — — — 0.3 — — 0.2 — — — 88 96 100 68 96 99 11 1.0 — — 0.2 — — — 0.3 — 0.2 — — — — 84 98 100 65 95 98 12 1.0 — — — 0.3 — — 0.3 — — — 0.3 — — 81 97 100 68 96 98 13 1.0 — — — — 0.2 — 0.5 — — — — 0.3 — 86 96  98 64 96 98 14 1.0 — — — — — 0.2 0.5 — — — — — 0.3 92 96  99 67 94 99 15 — 2.0 0.3 — 0.1 — — — 0.5 — 0.2 — — — 89 97  98 71 94 99 16 — 2.0 — 0.3 — 0.1 — — 0.5 — — 0.3 — — 87 96  99 65 94 99 17 — 2.0 — — 0.1 — 0.3 — 1.0 — — — 0.3 — 90 97  99 70 95 99 18 — 2.0 0.2 — — — 0.2 — 1.0 — — — — 0.3 91 95  98 71 93 99 Comparative  2 1.0 — 0.2 — — — — — — — — — — — 91 96 100 48 92 96 Examples  3 — 2.0 0.3 — 0.1 — — — — — — — — — 93 96 100 45 93 98  4 1.0 — — 0.2 — — — — — — — — — — 88 98 100 46 96 99

It is appreciated from Table 2 that although the initial NO_(x) conversion performance of the present catalysts was not as good as that of the comparative examples, the ratios of the NO_(x) conversion performance after the durability test to the initial NO_(x) conversion performance (the NO_(x) conversion performance after the durability test/the initial NO_(x) conversion performance) of the present catalysts were superior to those of the comparative catalysts by far. That is to say, the present catalysts had NO_(x) conversion performance less deteriorated by the durability test, and were superior in durability to the comparative catalysts.

Having now fully described the present invention, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made thereto without departing from the spirit or scope of the present invention as set forth herein including the appended claims. 

What is claimed is:
 1. A catalyst system for purifying exhaust gases, comprising, being arranged in an order from the upstream to the downstream of exhaust gas flow, a first catalyst consisting of a noble metal catalyst ingredient and at least one metal selected from the group consisting of nickel (Ni), iron (Fe), zinc (Zn) and vanadium (V) loaded on a porous acidic support of ZrO₂, a second catalyst in which at least one NO_(x) absorber selected from the group consisting of alkali metals, alkaline-earth metals, and rare earth metals is loaded on a porous support, and a third catalyst in which a noble metal catalyst ingredient is loaded on a porous support.
 2. A catalyst system for purifying exhaust gases according to claim 1, wherein in said second catalyst, a transition metal is loaded on said porous support.
 3. A catalyst system for purifying exhaust gases according to claim 1, wherein said second catalyst consists of the at least one NO_(x) absorber loaded on the porous support.
 4. A catalyst system for purifying exhaust gases according to claim 1, wherein in said second catalyst, said porous support comprises a material selected from the group consisting of Al₂O₃, zeolite, SiO₂, ZrO₂, and SiO₂—Al₂O₃.
 5. A catalyst system for purifying exhaust gases according to claim 1, wherein in said third catalyst, said porous support comprises a material selected from the group consisting of Al₂O₃, zeolite, SiO₂, ZrO₂, and SiO₂—Al₂O₃.
 6. A catalyst for purifying exhaust gases according to claim 2, wherein said transition metal is a composite oxide. 